Interaction Between Resistant And Susceptible Apple Cultivars And Colletotrichum Fructicola

Glomerella Leaf Spot（GLS） is a new epidemic disease in apple-producing area, and the main pathogen is Colletotrichum fructicola. Different from apple bitter rot, which only disserves fruits, GLS occurrs on apple leaves. What’s more, there was significant difference between cultivars, GLS caused leaves necrosis in early fall of growing season on Gala, Golden Delicious, and other Golden Delicious varieties series. However, cv. Fuji, cv. Red Star and other cultivars were not infected. In this study, in order to reveal the infection characteristics of C. fructicola on different cultivars and the defense mechanism of resistant cultivars, we investigated the interaction between C. fructicola and the resistant cv. Fuji and the susceptible cv. Gala by cytological and histochemical methods, including the producing of H₂O₂ and papilla which reflect the host defense response. There are the main findings:1. The inoculation system of GLS in vitro was established: Wound inoculation was conducted on excised expanded leaves with mycelia disks, and incubated in moisture at 28℃, then investigated the size of necrosis, 4 dai. This method was simple with easily available material and controlled conditions, and hoped could be used to evaluate pathogenicity of GLS accurately and quickly.2. The pathogenicity of C. fructicola varied with apple cultivars and inoculation methods: C. fructicola showed no pathogenicity with non-wound inoculation using mycelia disks; but showed high pathogenicity with wound inoculation and resulted rounded brown necrotic spots; with inoculation of spore suspension, C. fructicola only infected the susceptible cv. Gala, and showed different symptoms from those resulted from mycelia disks inoculation, necrosis was reddish-brown and rounded or irregular. It showed that C. fructicola invaded a host only by conidia germination and forming appressoria, hyphae were not direct invaded hosts.3. Cytological observations showed that the percentage of conidia germination and appressoria formationin cv. Fuji were decreased compared with in cv. Gala, and the infection vesicles and primary hyphae formation on the susceptible cultivars were significantly inhibited, less than 2.5% in the susceptible varieties. Furthermore, secondary hyphae, which werere garded as a conversion character from biotrophic to necrotrophic of Colletotrichum, were not discovered in the resistant cv. Fuji. This result suggested C. fructicola was hemi-biotrophic on the susceptible cv. Gala, and latent-infection in the resistant cv. Fuji. Resistant cultivars may have some structures or factors to prevent the infection structure forming of C. fructicola.4. The results of histochemical staining showed that C. fructicola induced the accumulation of H₂O₂ and the formation of papillae. And the both interactions were increasing with the infection time. The production of H₂O₂ and papillae in the resistant cv. Fuji were significantly higher than that in the susceptible cv. Gala. H₂O₂ was mainly produced around the penetration sites and in the walls of infected cells. It also found that in the early infected process, SPC was formed mainly, then SPC decreased and LPC increased. The results showed that the accumulation of H₂O₂ and the formation of papillae may play an important role in the defense responses of apples.